1. Field of the Invention
This invention relates to an adaptive electronic hybrid circuit for use in a two-wire to four-wire connection in a telephone transmission line.
2. Description of the Prior Art
A commercial telephone transmission line generally consists of a two-wire circuit in the vicinity of each subscriber and a four-wire circuit for long-distance link, with hybrid circuits used for connecting them.
A conventional hybrid circuit consists of a transformer as shown in FIG. 4 of the article titled "The Effects of Time Delay and Echoes on Telephone Conversations" by J. W. Emling et al. published in The Bell System Technical Journal (November issue 1963), pp. 2869-2891 (Reference 1).
On the other hand an electronic hybrid circuit has been proposed, in which an active element is used instead of a transformer. For details reference is made to FIG. 5 of the paper titled "DSS-1, A Digital Local Switching System with Remote Line Switches" by C. G. Svala published in National Telecommunication Conference 1977, pp. 39:5-1-5-7 (Reference 2).
These hybrid circuits, however, require the terminal impedance or the balance impedance to be well matched with the impedance of the two-wire circuit. Inadequate matching between these impedances, as is well known, results in such phenomena as singing, oscillation or echoes, each of which disturbs telephone conversation. Therefore, particular care should be taken in matching the terminal impedance. However, because the impedance of the two-wire circuit varies with its length and the terminal impedance at its other end, the impedances of two-wire circuits differ from one subscriber circuit to another. Accordingly, termination with a fixed impedance may result in considerable mismatching.